In recent years, a motor has been employed not only in trains but also in cars. For example, a hybrid car, which has attracted attention because of its capability of reducing exhaust gas and preventing environmental pollution and has been commercialized, is driven by a gasoline engine and a motor as power sources. A voltage supplied to such a motor for cars is conventionally about 200 V to 300 V. In recent years, however, the supply voltage has been increased so as to enhance the accelerating force for cars. It is expected that a supply voltage of as high as about 500 V to 1000 V will be mainstream in the future.
To cause a battery to generate the same voltage as the supply voltage, a considerably large number of unit batteries need to be connected in series, leading to a large size of the whole battery. Therefore, for example, a technique of providing a boost chopper circuit in a voltage converting device or the like to boost a direct-current voltage obtained from a battery, has been employed (see, for example, Patent Document 1).
The boost chopper circuit comprises a reactor L, a capacitor C, and a switching device capable of switching large power, such as an insulated gate bipolar transistor (IGBT) or the like. The switching device is driven by a drive signal of about 20 kHz or less output from a switching device drive circuit based on a pulse signal, for example. The boost voltage is controlled based on a pulse width (duty ratio) of the pulse signal.
Also, when the boost chopper circuit is used, protection against overcurrent during the start of a motor or the like is facilitated by a known technique of allowing a switching device to perform a gentle switching operation by increasing the gate resistance of the switching device or reducing the gate voltage (see, for example, Patent Document 2).
There is also a technique of driving a switching device, in which an operation state of the switching device is detected, and the switching device is controlled, depending on the detected operation state. Hereinafter, these techniques will be described with reference to Patent Documents 3 and 4.
Examples of a switching device that performs a switching operation based on a pulse signal (generally called PWM drive) include a field effect transistor (hereinafter referred to as a MOSFET), an insulated gate bipolar transistor (hereinafter referred to as an IGBT), and the like. These switching devices are characterized by a relatively high switching operation speed, and are widely employed so as to achieve a high-frequency inverter and converter. In recent years, the voltage, current and speed of these switching devices have been rapidly increased, but conversely, the high-frequency and high-speed switching operation causes a surge voltage that may break down the switching device or generate noise that interferes with other electronic devices.
Here, the configuration of a general switching device drive circuit is shown in FIG. 20. In FIG. 20, the output of a PWM signal circuit 3 is connected to the input of a switching device drive circuit 200, and the output of the switching device drive circuit 200 is connected to the control terminal of a voltage drive switching device 1. The switching device drive circuit 200 comprises a drive signal output circuit 400, an ON-control voltage circuit 500, an OFF-control voltage circuit 600, and a control resistor 700. The ON-control voltage circuit 500 and the OFF-control voltage circuit 600 output constant voltages sufficient to turn ON and OFF the voltage drive switching device 1, respectively. The drive signal output circuit 400 is operated in synchronization with a PWM signal output from the PWM signal circuit 3, and outputs a power-amplified signal via the control resistor 700 to the control terminal of the voltage drive switching device 1.
Next, operation waveforms of the switching device drive circuit 200 and the voltage drive switching device 1 of FIG. 20 are shown in FIG. 21. In FIG. 21, a PWM signal, a drive signal, an ON-control voltage, an OFF-control voltage, a drain current, and a drain voltage are an output signal of the PWM signal circuit 3, an output signal of the switching device drive circuit 200, an output voltage of the ON-control voltage circuit 500, an output voltage of the OFF-control voltage circuit 600, and a drain voltage and a drain current of the voltage drive switching device 1, respectively.
Initially, the PWM signal circuit 3 outputs the PWM signal for turning ON/OFF the voltage drive switching device 1. The PWM signal is instantaneously switched from the Low level to the High level at time t1 in FIG. 21, and from the High level to the Low level at time t5. Next, the switching device drive circuit 200 power-amplifies the PWM signal, and inputs the resultant signal as a drive signal to the control terminal of the voltage drive switching device 1. The drive signal and the PWM signal are synchronous.
An operation of turning ON the voltage drive switching device 1 will be described in detail. Although the PWM signal goes to the High level at time t1, the rising of the drive signal is delayed to time t2 due to a delay caused by a circuit in the switching device drive circuit 200 or a current limitation caused by the control resistor. From time t2, charging of a gate-source capacitance (not shown) of the voltage drive switching device 1 is started, so that the drive signal (i.e., the voltage of the control terminal) gradually increases. At time t3, the voltage drive switching device 1 reaches its threshold voltage, so that the voltage drive switching device 1 is turned ON, thereby starting charging of a drain-gate capacitance (not shown) in addition to the gate-source capacitance of the switching device 1. By this operation, the drive signal is clamped in the vicinity of the threshold voltage (this operation is called a mirror effect during turning ON; from time t3 to time t4 in FIG. 21). When charging is substantially completed, the drive signal gradually increases again from time t4 and then reaches an ON-control voltage that is output by the ON-control voltage circuit 500, and ends the turning-ON operation. In the voltage drive switching device 1, a drain current starts flowing from time t3 and a drain current flows at time t4.
Next, an operation of turning OFF the voltage drive switching device 1 will be described in detail. At time t5, the PWM signal goes to the Low level, but as in the turning-ON operation, the falling of the drive signal is delayed to time t6 due to a delay caused by a circuit in the switching device drive circuit 200 or a current limitation caused by the control resistor. From time t6, discharging is started from the gate-source capacitance and the drain-gate capacitance of the voltage drive switching device 1, so that the drive signal (i.e., the voltage of the control terminal) gradually decreases. At time t7, the drive signal reaches the threshold voltage of the voltage drive switching device 1, so that the voltage drive switching device 1 is turned OFF, and only the gate-source capacitance continues to be discharged. By this operation, also as in the turning-ON operation, the drive signal is clamped in the vicinity of the threshold voltage (this operation is called a mirror effect during turning OFF; from time t7 to time t8 in FIG. 21). When discharging is substantially completed, the drive signal gradually decreases again from time t8 and then reaches the OFF-control voltage output by the OFF-control voltage circuit 600, and ends the turning-OFF operation. In the voltage drive switching device 1, the drain voltage starts increasing from time t7, and then reaches a desired drain voltage at time t8.
Here, when the change rate of the drain current (di/dt) and the change rate of the drain voltage (dv/dt) are steep, a surge current and a surge voltage occur as shown in portions A and B, respectively, disadvantageously leading to breakdown of the switching device or an interference with other electronic devices due to noise.
To solve these problems, a technique of reducing the operating speed of a switching device by changing the amounts of currents flowing into and out of the control terminal of the switching device, depending on the operation state of the switching device, has been conventionally proposed.
According to Patent Document 3, an IGBT is provided with a main emitter terminal through which a collector current mainly flows, and an auxiliary emitter terminal through which a small auxiliary emitter current proportional to the main collector current is extracted. The main emitter terminal and the auxiliary emitter terminal are connected together via an inductance. Thereby, the starting time of rising of the main collector current is detected, and the amount of a current flowing into the control terminal of the switching device is reduced and changed between before and after the main collector current flows out. As a result, the current change rate (di/dt) of the main collector current can be caused to be gentle. A reduction in current change rate (di/dt) of the main collector current leads to a reduction in voltage change rate (dv/dt) of the main collector voltage. Thus, the surge voltage and the switching noise can be suppressed.
According to Patent Document 4, the control circuit of the switching device included in an inverter comprises a voltage detecting unit for detecting the voltage of a direct-current power supply supplied to the inverter. The amount of a current flowing into the control terminal of the switching device is changed so that the operating speed of the switching device varies depending on the detected voltage. Thereby, when the voltage of the direct-current power supply is low, the value of a surge voltage due to the switching operation is also low along with the voltage of the direct-current power supply, so that the operating speed of the switching device can be increased. Therefore, when the voltage of the direct-current power supply is low, a switching loss due to the switching operation can be reduced without occurrence of an excessive surge voltage.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2005-354763
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2001-268926
Patent Document 3: Japanese Unexamined Patent Application Publication No. H10-32976
Patent Document 4: Japanese Unexamined Patent Application Publication No H09-23664